This invention relates to electrophoretic display devices and compositions as generally disclosed in Ota, U.S. Pat. No. 3,668,106, U.S. Cl. 204/299, hereafter the '106 patent.
In such a device, a colloidal suspension layer, comprising electrophoretically-active particles in a dielectric medium, is placed between two electrodes, at least one of which is transparent. The particles are chosen to optically contrast with the medium; e.g., light-reflecting vs. light-absorbing. Thus, if the particles are white and the medium is black, the suspension will be gray until the application of an electric field. Then, the particles affected by the field will electrophoretically move to one electrode giving a white appearance with the corresponding areas of the other electrode appearing black.
A great number of materials can be used as the electrophoretic particles and suspending media; also additives such as charge control agents and dispersants can be effectively utilized. A good summary of such typical materials appears in the '106 patent, and to that extent, the disclosure therein is incorporated here by reference.
There are a number of display cell configurations comprising a housing, two spaced-apart electrodes and electrophoretic suspension there-between. Again, that portion of the '106 patent including the drawings relating to cell construction is incorporated here by reference.
The problem addressed by this invention relates to the settling-out of particles from the suspending medium during periods of inactivity. Ideally, if a display cell is to be available for informational use on demand, the particles should be relatively uniformly dispersed in the medium with no field applied. If settling out occurs, uneven image contrast may result if the particles collect in one portion of the cell. Because of the desire to provide maximum optical contrast between particle and medium, particles with weight densities greater than that of the suspending medium are usually employed. For example, titanium dioxide is a preferred white pigment for use as the electrophoretic particle, and hydrocarbons such as methanol are good suspending media. The density of TiO.sub.2 is 4.2 grams per cubic centimeter, and methanol's is 0.8. Generally, the most suitable suspending media have refractive indices of about 1.4.+-.0.1 and densities between 0.8 and 1.3 gm/cm.sup.3, and electrophoretic pigment particles have refractive indices between 1.5 and 2.9 and densities greater than 2.2. Since it is known that the velocity of settling is directly proportional to the difference between the particle density and the medium density, it follows that heavier particles will tend to settle out. Ideally, the density of the particles should be about equal to that of the medium to give a settling velocity of zero or near zero.